I brewed a brown ale with a gallon of maple syrup and it came out awesome! I say go for it!!
Never made a beer with maple syrup but i have made mead and cider and have found the flavor is hard to keep through fermentation. Maybe make a dry beer and add it after to keep the maple flavor?
Last year at Brass City there was a guy talking about the history of cider, he flavored a batch with Maple Water, which he described as the thin sap that comes from the tree but is not yet cooked down into cider. I don't remember why it was better to use that form, but it did impart a somewhat subtle maple flavor.